Probeless DC testing of CMOS I/O circuits

ABSTRACT

A method and implementation is described by which I/O input and output circuitry of a CMOS chip are measured without the need to probe the chip. Output driver transistors are used to provide marginal voltages to test input circuits, and the output driver transistors are segmented into portions where a first portion is used to provide a representative “on” current, which is coupled to a test bus that is further connected to a current comparator circuit contained within the chip. Both leakage and “on” current of the driver transistors is measured using segmented driver transistors. The output of the current comparator circuit is connected to a test scan register or to a test output from which test results are obtained digitally. The testing techniques are also applicable for other semiconductor devices.

This is a Divisional Application of DS05-011, Ser. No. 11/366,742, filed on Mar. 2, 2006, and incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention is related to semiconductor testing, and in particular to testing CMOS I/O drivers.

2. Description of Related Art

As the integration of semiconductor chips continues, smaller and smaller geometries have occurred with more and more I/O circuits included to reduce the number of chips needed to implement a function. With the onset of advancement in displays creating LCD screens and other methods of display, the need for testing the ability of the I/O drive circuits to perform a specified function is necessary to guarantee the quality of the drivers. Driver leakage current testing as well as drive current capability is crucial to guarantee a quality display to insure segment and common connections to a display create a quality image.

A paper title “Verification and Auto Test for LCD Driver/Controller”, Zhuo et al., Proceedings 5th International Conference on ASIC, October 2003, Vol.: 2, pp.: 1175-1178 is directed to two methods that fulfill the voltage measurements for all terminals of an LCD driver using several A/D converters. U.S. Pat. No. 6,774,656 (Baur, et al.) is directed to a self-test for leakage current of driver/receiver stages. U.S. Pat. No. 6,725,171 (Baur, et al.) is directed to a self-test structure with a split, asymmetric controlled driver output stage. U.S. Pat. No. 6,262,585 (Frodsham, et al.) is directed to an apparatus for I/O leakage self-test in an integrated circuit.

In FIG. 1A is shown a driver circuit 10 of prior art, which comprises a driver logic 11, a P-channel transistor M1 and an N-channel transistor M2. The drain of transistor M2 is connected to the drain of transistor M1, which is further, connected to a chip I/O pad. A bias voltage +V is connected to the P-channel transistor M1 and a bias voltage −V is connected to the source of the N-channel transistor M2. The driver logic 10 controls the gates of M1 and M2 to produce at the I/O pad an output voltage and current to necessary to drive an LCD display or other electronic equipment. Testing of the driver circuit 10 requires probing of the I/O pad with a tester while manipulating the driver logic 11 to produce the required test conditions.

In FIG. 1B is shown a driver circuit 15 of prior art, which comprises the driver logic 16 controlling the gates of a first P-channel transistor M1, a second P-channel transistor M3 and an N-channel transistor M2. The drains of transistors M1 and M3 are connected to the drain of transistor M2 and further connected to a chip I/O pad. Transistor M1 is biased with a voltage +V, transistor M2 is biased with a voltage −V and transistor M3 is biased with a voltage +Vm. Testing of the driver circuit 15 requires probing of the I/O pad with a tester while manipulating the driver logic 16 to produce the required test conditions.

The probing of the small I/O pads of an integrated circuit chip by a tester exposes the I/O pad to damage, which creates problems for electrically connecting the I/O pads to the next higher level of assembly. A design and method is required to eliminate this exposure and still provide a means for efficiently testing the quality of the I/O drivers including leakage and drive current.

SUMMARY OF THE INVENTION

It is an objective of the present invention to provide an implementation and method to allow testing of an I/O driver circuit and I/O receiver circuit without the probing of a semiconductor chip I/O pad to which the drive circuit is connected.

It is also an objective of the present invention to segment the gate of the transistors forming I/O driver circuit into two or more portions, one of which is used to create a test transistor to produce a representative driver output load current at a reduced current level.

It is further an objective of the present invention to control all gate segments of each driver transistor to produce current in normal operation.

It is also further an objective of the present invention to measure leakage current and drive current of CMOS transistors forming an I/O driver circuit using a current comparator circuit within an integrated circuit chip containing the I/O drive circuit.

It is still further an objective of the present invention to connect the output of the current comparator to a primary output (e.g. by multiplexing) or to a test scan register to allow scanning out driver current test results.

It is also still further an objective of the present invention to couple a plurality of I/O driver circuits within a semiconductor chip to a test bus contained within the semiconductor chip and switch test current from each transistor of the I/O driver circuits to the test bus and a current comparator circuit connected to the test bus.

It is another objective of the present invention to connect the test current to the test bus using a switch circuit connected to each chip output pad connected to an I/O driver circuit.

In the present invention the control gates of CMOS output driver transistors are segmented into two or more portions forming two or more transistors sharing the same semiconductor device channel. The first of the portions is used as a test transistor to provide a means by which the drive current of the CMOS driver can be tested. The other portions are controlled with the first portion to produce drive current in the regular operation of the driver transistor. The ratio of the first portion (the test transistor) to the other portions is any suitable ratio that keeps the test current within acceptable limits but high enough to provide sensible results. The portions of the segmented control gate can be grouped in various ways, for example interleaving the first portion (test portion) with the other portions so as to provide an approximate even distribution of the portions over the total surface of driver transistor control gate area or grouped in clusters of segmented elements forming the control gate portions.

The CMOS driver is comprised of a P-channel transistor and an N-channel transistor connected together to an output pad of a semiconductor chip. The P-channel transistor is biased with a positive voltage and the N-channel transistor is biased with a negative voltage. Both the P-channel driver and the N-channel driver are made with segmented control gates where the test transistor portion of each is controlled by a test logic through a selector circuit. During normal operation driver logic controls all segments of the control gates through the selector. Additional segmented driver transistors biased by the same or different voltages can be added to the driver circuit and control in a similar fashion by the driver logic and test logic through a selector circuit.

A switch controlled by the test logic connects the output pad of the chip to a test bus that is connected to the current measuring circuit. The current measuring circuit is a current comparator circuit, which couples the compare results to a test scan register. All the output driver circuits connected to an I/O pad are further connected from the I/O pad to the current comparator circuit by a switch. Selecting a switch coupled to an output driver connects the output driver to the current comparator circuit. A test pad connected to the test bus can be used to allow a tester to measure test current form the output drivers.

Leakage current from the P-channel driver transistor or the N-channel driver transistor of an output drive circuit is measured by biasing each driver transistor separately and closing the switch connecting a particular drive circuit to the test bus. then the current comparator or tester connected to the test pad is used to measure the leakage current. If all switches connected to the test bus are closed simultaneously, a collective output driver leakage current from the N-channel transistors or the P-channel transistors can be measured. If the collective leakage is below a predetermined amount, further testing of the individual driver circuits for leakage is not necessary.

Drive current from the P-channel driver transistor, or the N-channel driver transistor, is accomplished by biasing the particular driver transistor and controlling one of the control gate test segments to be on, closing the switch connecting the I/O pad to the test bus and comparing the resulting current with the current comparator, or by measuring the resulting current with a tester connected to a test pad connected to the test bus.

Another way to couple drive current from the P-channel driver transistor, or the N-channel driver transistor, is to bias the particular driver transistor or one of its segments, reconfigure (by switches) a portion or the whole compliment transistor as the sensing transistor of a current mirror, and connect the sensing transistor to a mirror transistor with a low current switch device. The corresponding mirror transistor, which can be of much smaller size than the sensing transistor, couples a proportional current to a test bus to which the current comparator is connected.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention will be described with reference to the accompanying drawings, wherein:

FIG. 1A is a diagram of prior art of an I/O driver circuit;

FIG. 1B is a diagram of prior art of an I/O driver circuit containing an additional driver transistor and bias voltage;

FIGS. 2A and 2B are diagrams of an I/O driver circuit of the present invention;

FIGS. 2C and 2D are diagrams of a segmented driver transistor gate of the present invention;

FIGS. 2E and 2F are diagrams of a current comparator of the present invention;

FIG. 3A is a diagram of the present invention showing a plurality of I/O driver circuits connected to a current comparator circuit;

FIG. 3B is a diagram of the present invention showing a variety of circuitry connected to a current comparator circuit;

FIGS. 4A and 4B are diagrams of the preferred embodiment of the present invention of a current mirror circuit to couple driver transistor current to a current comparator circuit;

FIGS. 4C and 4D are diagrams of an alternate implementation to the preferred embodiment of the present invention of a current mirror circuit that couples driver transistor current to a current comparator circuit

FIG. 4E is a diagram of a test configuration of the present invention for I/O input circuits;

FIG. 4F is a voltage level diagram of the present invention for I/O input circuits;

FIGS. 5A and 5B are flow diagrams of the present for the method of testing leakage current of the I/O driver transistors;

FIGS. 6A and 6B are flow diagrams of the present invention for the method of testing the test transistor segments of the I/O driver transistors; and

FIG. 7 is a flow diagram of the present invention to test input circuits by coupling marginal voltage levels to input circuits from output driver circuits.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 2A is shown a circuit diagram of an I/O receiver circuit 18 coupled to logic circuits 19 and an output driver circuit 20 coupled to an I/O pad 23. The output driver circuit 20 is coupled by a switch 21 to a test bus 22. The output driver 20 comprises a P-channel transistor M1 and an N-channel transistor M2. The control gates of transistors M1 and M2 are segmented into a plurality of control gate portions forming a plurality of transistors sharing the same transistor device channel. For simplicity of explanation the segmentation of the transistors M1 and M2 is assumed to be two portions. Thus, the control gate of M1 is shown to be segmented into two portions forming transistor M1 a and M1 b, in like manner the control gate of M2 is shown to be segmented into two portions forming transistors M2 a and M2 b. By definition herein transistors M1 b and M2 b are test transistors, and M1 b and M2 b have a gate area that is a fraction of the total gate area of M1 a plus M1 b and M2 a plus M2 b, respectively, FIG. 2C shows an example of a control gate layout comprising an interleave of the two portions of the control gate metallization, where metallization 40 represents the control gate for transistors M1 b and M2 b and metallization 41 represents the control gate for transistors M1 a and M2 a.

In FIG. 2D is shown a preferred example of a segmented control gate where each finger of the control gate 40 is individually connected 42 to a selector circuit S1 or S2 shown in FIG. 2A. The selector circuit has the capability of configuring the control gate segmentation to accommodate the test requirements of a particular driver test by selecting a single finger (element) or any combination of fingers to create the test transistors M1 b and M2 b. In normal operation of the driver circuit the selector circuits S1, S2 and S3 select all of the fingers to form the control gates of driver transistors M1, M2 and M3.

Continuing to refer to FIG. 2A, the sources of the P-channel transistors M1 a and M1 b are connected to a +V bias voltage, and the sources of the N-channel transistors M2 a and M2 b are connected to a −V bias voltage, where in the preferred implementation the voltage −V is circuit ground. The drains of M1 a and M1 b are connected to the drains of M2 a and M2 b, and further connected to a semiconductor chip I/O pad 23. Connected to the chip I/O pad 23 is a switch circuit 21, which when closed, couples current from the output driver 20 to the test bus 22. The test bus 22 couples I_(test) to a current comparator circuit 24 where the current comparator circuit 24 compares I_(test) to a reference current I_(ref). It should be noted that the direction of the currents I_(test) and I_(ref) are indicated for currents for a positive voltage bias used for driver transistor M1 and would be in opposite direction for a negative voltage bias used for driver transistor M2. The output of current comparator 24 is connected to a scan in port of a test scan register 26 that has a scan out connection to be couple to a tester (not shown). Alternatively, the output of the current comparator circuit 24 can be made accessible to a tester or to built-in self test (BIST) logic.

The control gates of transistors M1 a and M1 b receive a control gate voltage from selector circuit S1, and in like manner transistors M2 a and M2 b receive a control gate voltage from selector circuit S2. A test select signal is coupled to selector circuits S1 and S2 to enable the selector circuits S1 and S2 to allow the control gates of M1 b and M2 b, respectively, to be controlled by the test logic circuit 27, and to disable the control gates of M1 a and M2 a. A driver logic circuit 28 controls the selectors S1 and S2 to apply the appropriate control gate voltage during normal operation of the output driver 20 to transistors M1 and M2, respectively. During normal operation control gates of M1 a and M1 b are controlled together to produce an output current from M1 and in like manner control gates of M2 a and M2 b are controlled together to produce and output current from M2.

The test logic circuit 27 controls the switch 21 closed during test of the output driver circuit 20 allowing current from the output driver circuit to be coupled to the test bus 22. During normal operation of the output driver circuit 20 the test logic circuit opens the switch 21 isolating the test bus 22 from the chip I/O pad 23.

In FIG. 2B is shown an I/O driver circuit 20 that contains an additional P-channel transistor M3 biased to a positive voltage Vm where the segmented gates of transistor M3 are controlled through a selector circuit S3. The control gate of M3 is segmented into two portions to form transistors M3 a and M3 b similar to that previously discussed with respect to M1 in FIG. 2A. The test logic 27 controls the control gate transistor M3 b to produce a drive current I_(Test) that is connected to the test bus 22 when the switch circuit 21 is closed by the test logic 27. Similar to the P-channel transistor M1, the P-channel transistor M3 is biased to a voltage VM and leakage current from M3 is coupled to the test bus 22 for measurement by switch 21. For drive current measurement transistor M3 b is controlled to be on, and the transistor drive current I_(Test) is coupled to the test bus 22 by the switch 21.

In FIG. 2E is shown a schematic diagram of an implementation of the current comparator circuit 24 comprising a current mirror circuit 30, a capacitance C and an inverter 31. The current mirror circuit 30 is formed with N-channel transistors and produces a current Ir1 that is the mirror of I_(ref). A test current I_(Test) from a P-channel driver transistor M1 (FIG. 2A) is connected to the current comparator circuit through the closed switch 21 and combines with Ir1 to charge capacitance C. The capacitance C can be either a capacitor device or parasitic capacitance of the devices connected to the test bus. When I_(Test) is smaller in magnitude than Ir1, the capacitor C is charged with a negative potential, which is inverted to a positive voltage by the inverter circuit 31 and coupled to the scan in of the test scan register 26. When I_(Test) is larger in magnitude than Ir1, the capacitor is charged to a positive potential and the inverter circuit 31 couples a negative scan in voltage to the test scan register. The inverter circuit 31 is designed with sufficient gain so as control the amount of uncertainty with respect to which current is marginally larger in magnitude to within allowable test margins.

In FIG. 2F is shown a schematic diagram of an implementation of the current comparator circuit 24 comprising a current mirror circuit 32, a capacitance C and an inverter 31. The current mirror circuit 32 is formed with P-channel transistors and produces a current Ir2 that is the mirror of I_(ref). A test current I_(Test) from an N-channel driver transistor M2 (FIG. 2A) is connected to the current comparator circuit through the closed switch 21 and combines with Ir2 to charge capacitance C. The capacitance C can be either a capacitor device or parasitic capacitance of the devices connected to the test bus. When I_(Test) is smaller in magnitude than Ir2, the capacitor C is charged with a positive potential, which is inverted to a negative voltage by the inverter circuit 31 and coupled to the scan in of the test scan register 26. When I_(Test) is larger in magnitude than Ir2, the capacitor is charged to a negative potential and the inverter circuit 31 couples a positive scan in voltage to the test scan register. The inverter circuit 31 is designed with sufficient gain so as control the amount of uncertainty with respect to which current is marginally larger in magnitude near the value of I_(ref) to within allowable test margins.

In FIG. 3A is shown a plurality of N I/O driver circuits 20 connected to the test bus 22 by a plurality of switch circuits 21. A test pad 45 is connected to the test bus 22 to allow a tester to measure the current I_(Test). The use of the tester can be in place of the current comparator circuit 24 or as a means to verify the results of the current comparator circuit. In the case of leakage current, the plurality of switches 21 can all be closed simultaneously to measure a composite leakage current from either the P-channel driver transistors M1 or the N-channel driver transistors M2 shown in FIG. 2A and the additional P-channel driver transistor M3 shown in FIG. 4. The composite leakage current measurement, made by either the current comparator 24 or the tester, allows a quick assessment of I/O driver circuit leakage to determine if individual I/O driver circuits 20 require additional leakage current measurement.

In FIG. 3B is an example of a variety of circuitry 20, 50, 51, 52 and 53 that is coupled to a current comparator 24. The circuits shown in FIG. 3B are coupled to a current comparator circuit 24 through a switch circuit 21. Depending upon design requirements, the circuits 20, 50, 51, 52 and 53 may be located on the same integrated circuit chip, or on separate integrated circuits chips where each separate chip contains a current comparator circuit 24. As described with respect to FIG. 2A and FIG. 2B, the I/O driver circuit 20 is coupled to a switch circuit 21 that is connected to the chip I/O pad 23 to which the I/O driver circuit is connected. The switch 21 is controlled by a test logic circuit 27 to close the switch, which allows a test current I_(Test) from the I/O driver to be connected to the test bus 22. The test current I_(Test) can be leakage current or an on current from circuitry within the I/O driver circuit. The current comparator circuit 24 compares I_(Test) to a reference current I_(ref) and couples the compare results to a test scan register 26 or other readout mechanisms such as a tester and BIST logic. A test pad 45 allows a tester to couple to the test bus 22 to allow a tester to make test measurements.

Continuing to refer to FIG. 3B, in circuit 50 an open drain P-channel transistor device M4 is connected to a chip I/O pad 23. A switch 21 controlled by the test logic 27 is connected to the chip I/O pad to couple a test current I_(Test) from M4 to the test comparator circuit 24. The driver logic 28 controls the transistor device M4 in an off state to produce a leakage current to be coupled to the current comparator 24 and controls M4 to be in an on state to produce an on current to be coupled to the current comparator. The reference current I_(ref) is set to a value of current necessary to measure leakage current and then set to a value of current necessary to measure on current of M4.

Continuing to refer to FIG. 3B, in circuit 51 an open drain N-channel transistor device M5 is connected to a chip I/O pad 23. A switch 21 controlled by the test logic 27 is connected to the chip I/O pad to couple a test current I_(Test) from M5 to the test comparator circuit 24. The driver logic 28 controls the transistor device M5 in an off state to produce a leakage current to be coupled to the current comparator 24 and controls M5 to be in an on state to produce an on current to be coupled to the current comparator. The reference current I_(ref) is set to a value of current necessary to measure leakage current and then set to a value of current necessary to measure on current of M4. The direction of I_(Test) and I_(ref) shown in FIG. 3B is symbolic and will be in a reverse direction for a negative source voltage as is the case with M5.

Continuing to refer to FIG. 3B, an impedance 33 is located between a driver circuit 32 and the chip I/O pad 23 to which is connected switch 21. The driver circuit 32 may comprise transistor devices M4, M5 and the driver circuits of FIG. 2A and FIG. 2B. The impedance 33 can be a resistance, an inductance or some combination of resistance and inductance and including a capacitance.

Continuing to refer to FIG. 3B, the current measuring technique describe herein is applied to an internal circuit 53, wherein an output current of logic A 34 is connected to an input to logic B 35. A switch “A” 36 under the control of the test logic 30 is opened to allow current from logic “A” 34 to be diverted to switch 21 and coupled to the test bus 22 when switch 21 is closed. The current from the output of logic “A” 34 is then compared to I_(ref) in the current comparator circuit 24 and the results of the current comparison is coupled to the test scan register 26.

It should be noted that current from circuitry containing bipolar transistors and other devices not specifically described herein can be measured in the same or similar fashion as described herein with respect to CMOS devices and circuits. It should be also noted that the current measurement techniques noted herein are applicable to all semiconductor devices with or without a segmented control function similar to that described in FIGS. 2A, 2B, 2C and 2D.

The semiconductor switch 21 shown in FIGS. 2A and 2B must be of comparable size to the largest driver segmented test transistor M1 b and M2 b in order to connect the test current I_(test) to the test bus 22 without restricting the current. For each I/O that have the high current drivers this requires two relatively large additional transistors, which takes up valuable chip real estate to accommodate driver on-state current measurements.

In FIGS. 4A and 4B is shown a preferred embodiment of the present invention for measuring driver current in which the switch 21 a connecting driver current to the test bus 22 is implemented with a small, low current device. In FIG. 4A the P-channel segmented transistor M1 b is selected by the selector circuit S1 to be measured for on-state current and is biased with a +V bias. The N-channel segmented transistor M2 b is configured by the selector circuit S2 to be the input transistor to a current mirror circuit 47. The test logic 27 closes switch 21 b to produce a drain voltage, which when coupled to the gate of the N-channel transistor M6, produces a current I_(test) that is proportional to the current flowing in the segmented transistor M1 b and subsequently through the segmented transistor M2 b as a result of the mirror ratio between transistors M6 and M2 b. The switch device 21 a, which is closed by the test logic to couple the current mirror signal to the current mirror output transistor M6, is a minimal size device since little to no current is being carried by the switch device 21 a. Transistor device M6 must be of same length as M2 b, but the width of M6 can be chosen much smaller to reproduce a test current I_(test) of sufficient magnitude to provide an accurate current measurement. The drain, or output, of M6 is connected to the test bus 22, which couples I_(test) to the comparator circuit 24

In FIG. 4B the N-channel segmented transistor M2 b is selected by the selector circuit S2 to be measured for on-state current and is biased with a −V bias. The P-channel segmented transistor M1 b is configured by the selector circuit S1 to be the input transistor to a current mirror circuit 48. The test logic 27 closes switch 21 b to produce a drain voltage, which when coupled to the gate of the P-channel transistor M7, produces a current I_(test) that is proportional to the current flowing in the segmented transistor M2 b and subsequently through the segmented transistor M1 b as a result of the mirror ratio between M7 and M1 b. The switch device 21 a, which is closed by the test logic to couple the current mirror signal to the current mirror output transistor M7, is a minimal size device since little to no current is being carried by the switch device 21 a. Transistor device M7 must be of same length as M1 b but the width of M7 can be chosen to be much smaller to produce a useful test current I_(test) of sufficient magnitude to provide an accurate current measurement. The drain, or output, of M7 is connected to the test bus 22, which couples I_(test) to the comparator circuit 24. In FIGS. 4C and 4D is shown an alternative of the preferred embodiment of the present invention for measuring driver current in which the gates of the mirror transistors M6 and M7 are connected to the test bus 22. Thus, only one mirror transistor M6 and one mirror transistor M7 is required for coupling test current to the current comparator circuit 24 from all of the P-channel and N-channel test transistors M1 b and M2 b connected to the test bus. In FIG. 4C the P-channel segmented transistor M1 b is selected by the selector circuit S1 to be measured for on-state current and is biased with a +V bias. The N-channel segmented transistor M2 b is configured by the selector circuit S2 to be the input transistor to a current mirror circuit 47. The test logic 27 closes switch 21 b to produce a drain voltage, which when coupled to the gate of the N-channel transistor M6, produces a current I_(test) that is proportional to the current flowing in the segmented transistor M1 b and subsequently through the segmented transistor M2 b as a result of the mirror ratio between transistors M6 and M2 b. The switch device 21 a, which is closed by the test logic to couple the current mirror signal to the current mirror output transistor M6, is a minimal size device since little to no current is being carried by the switch device 21 a. Transistor device M6 must be of a same length as M2 b but its width can be chosen much smaller to reproduce a useful test current I_(test). It should be noted, that by connecting the test bus 22 to the gate of transistor device M6, only one M6 transistor device is required to accommodate current measurement of all P-channel driver transistors coupled to the test bus 22.

In FIG. 4B the N-channel segmented transistor M2 b is selected by the selector circuit S2 to be measured for on-state current and is biased with a −V bias. The P-channel segmented transistor M1 b is configured by the selector circuit S1 to be the input transistor to a current mirror circuit 48. The test logic 27 closes switch 21 b to produce a drain voltage, which when coupled to the gate of the P-channel transistor M7, produces a current I_(test) that is proportional to the current flowing in the segmented transistor M2 b and subsequently through the segmented transistor M1 b as a result of the mirror ratio between M7 and M1 b. The switch device 21 a, which is closed by the test logic to couple the current mirror signal to the current mirror output transistor M7, is a minimal size device since little to no current is being carried by the switch device 21 a. Transistor device M7 must be of same length as M1 b but the width can be chosen to be much smaller to reproduce a useful test current I_(test), It should be noted that by connecting the test buss 22 to the gate of transistor device M7, only one M7 transistor device is required to accommodate current measurement of all N-channel driver transistors coupled to the test bus 22.

In FIG. 4E is a configuration for testing an input circuit 18 connected to the same chip I/O pad 23 as a driver circuit and connected to an I/O pad 23 a not connected to a driver circuit. A driver circuit comprising transistor devices M1 and M2 and biased to voltages +V and −V is connected to a chip I/O pad 23. An I/O input circuit 18 biased to +V_(Logic) and −V_(Logic) is coupled to the same I/O pad 23 as the driver circuit. The output of the I/O input circuit 18 is couples input signals to chip logic 19. In a similar fashion a second I/O input circuit 18 a coupled to a second I/O pad 23 a is connected to the driver circuit containing transistor devices M1 and M2 through a switch 21.

FIG. 4F demonstrates input and output signal voltage levels. An operating voltage range for an up level signal is between a most positive up level (MPUL) and a least positive up level (LPUL). The operating voltage range for a down level signal is between a most negative down level (MNDL) and a least negative down level (LNDL).

Continuing to refer to FIG. 4E, the bias voltage for driver transistor M1 is set to a voltage equal the LPUL of an input signal that can be coupled the input circuit 18, and the selector circuit S1 selects transistor device M1 to connect the LPUL voltage to the input circuit 18. The logic 19 is polled to determine if the input circuit 18 was able to detect the LPUL voltage. The switch 21 is activated by the test logic 27 to couple the LPUL voltage to the second input circuit 18 a connected to a second I/O pad 23 a, and the logic 19 is polled to determine if the second input circuit 18 a was able to detect the LPUL voltage. In a like fashion the bias voltage for driver transistor M2 is set to a LNDL voltage of the I/O input circuits 18 and 18 a. The driver transistor M2 is selected by the selector circuit S2 and the logic 19 is polled to determine if the I/O input circuit 18 and 18 a were able to detect the LNDL voltage.

In FIG. 5A is a flow diagram of the present invention showing a method of testing leakage current of I/O driver transistors where a particular driver circuit transistor portion, either P-channel transistor M1 or M3 or N-channel transistor M2, is powered on for all the output driver circuits of a chip 60, controlling the transistor portion of the output driver circuits 20 (FIG. 2 and FIG. 4) to be off 61, and then closing the switch 21 coupling an output pad to the test bus 62. Coupling leakage current from the output driver circuit to a current comparator circuit 63. Compare the leakage current to a reference current 64, and couple the current comparator circuit results to a test scan register 65. If the output pad connected to an I/O driver circuit is not the last output pad 67 connected to an I/O driver circuit, then proceed to the next output pad 68, close the switch connecting the next output pad to the test buss 63, couple the leakage current from the next output pad to the current comparator 63, compare the leakage current of the next output pad to a reference current 64, couple the compare results of the leakage current from the next output pad to a test scan register 66. Open the switch connecting the next output pad to the test bus 66 and continuing until leakage current for the particular driver transistor connected to all output pads have been tested.

Continuing to refer to FIG. 5A, if the last pad has been tested 69, disconnect bias from the particular transistor portion of the output driver circuit 70, and if all output driver circuit transistor portions have been tested 72, then end the leakage testing. If all output transistor portions (M1, M2 or M3) of the output driver circuit have not been tested for leakage current 71, then connect a bias voltage to the next output transistor portion of the I/O drivers on the semiconductor chip 73. Control off the next output transistor portion 61, close the switch the output pad to the test bus 62, couple leakage from the next output transistor portion connected to the output pad to the current comparator circuit 63, compare the leakage current to a reference current 64, couple results of the current comparator circuit to a test scan register 65 and open the switch connecting the output pad to the test bus 66. Continuing in like fashion until all output pads for all driver circuit transistor portions have been tested for leakage current.

In FIG. 5B is a flow diagram of the present invention where all I/O driver portions, transistors M1, M2 and M3, connected to a particular chip I/O pad are tested for leakage current before going to a next chip I/O pad and performing leakage testing on the driver portions, transistors M1, M2 and M3, connected to the next I/O pad. A bias voltage is connected to a driver portion 80, for example +V connected to transistor portion M1 in FIG. 2. The transistor portion is controlled to an off state 81 and the switch coupling between the chip I/O pad and the test bus is closed 82. Leakage current is coupled from the driver connected to the I/O pad to a current comparator circuit connected to the test bus 83 where the current comparator compares the leakage current to a reference current 84. The results of the current comparator is coupled to a test scan register 85, the switch connected to the I/O pad is opened 86 and the bias connected to the transistor portion is disconnected 87.

Continuing to refer to FIG. 5B, if all driver portions connect to the I/O pad have not been tested for leakage current 88, a bias voltage is connected to the next driver portion 89. The next driver transistor portion is controlled in an off state 81 and the switch is connecting the chip I/O pad to the test bus is closed 82. Leakage from the next driver portion is coupled from the driver circuit to the current comparator 83, and the leakage current is compared to a reference current in the current comparator circuit 84. The results of the comparator circuit is coupled to a test scan register 85, the switch is opened 86, and the bias to the next driver portion is disconnected 87.

Continuing to refer to FIG. 5B, if all driver portions, comprising transistors M1, M2 and M3, connected to the I/O chip pad have been tested for leakage current 90 and the driver circuit connected to the last output pad has been tested for leakage current 91, then end the leakage testing. If the driver circuit connected to the next I/O pad has not been tested for leakage current 92, then go to the next I/O pad connected to a driver circuit 93 and connect a bias voltage to a driver portion 80, comprising transistors M1, M2 or M3, and control the selected driver portion connected to the next I/O pad in an off state 81. Close the switch connected between the next I/O pad and the test bus 82 and couple leakage current from the driver circuit to the current comparator circuit 83. Compare the leakage current from the next I/O pad to a reference current 84 coupling the results of the compare to the test scan register 85, opening the switch 86 and disconnecting the bias 87 to the driver portion. Continuing to test leakage current from the driver circuit portions until all driver portions connected to all I/O pads have been tested.

In FIG. 6A is a flow diagram of a method to test the drive current capability of the portions of an I/O driver circuit comprising transistors M1, M2 and M3. The transistors M1, M2 and M3 have a segmented gate, which forms a test transistor M1 b, M2 b and M3 b. FIG. 2D shows an example of the preferred configuration of a segmented gate where each segmented element 42 can be selected individually by the selector circuit S1, S2 and S3 to form a test transistor portion M1 b, M2 b and M3 b of the driver transistors M1, M2, and M3 respectively. Any combination of a plurality of segmented gate elements 42 can be configured by the selector S1, S2, and S3 to form the test transistor portion M1 b, M2 b and M3 b, depending on test requirements. Since the driver control gates are segmented to facilitate the testing of the driver, a driver transistor will not be completely tested until all segmented elements 42 or any combination thereof are configured as test transistor portions M1 b, M2 b, and M3 b and coupled to the current comparator circuit 24.

The test transistors M1 b, M2 b and M3 b are used to test the driver transistors M1, M2 and M3 respectively, at an appropriate current to test the integrity of the driver transistors M1, M2 and M3 area. The bias voltages, +V, −V and +Vm are connected to the driver transistors M1, M2 and M3, respectively 100. A test transistor of a driver transistor connected to a chip I/O pad is turned on 101 by the test logic, which closes the switch coupling the I/O pad to the test bus 102. Current from the test transistor portion of the driver transistor is coupled to a current comparator circuit, and the current is compared to a reference current connected to the current comparator 103. The results of the comparator circuit is coupled to an output mechanism 104 comprising readout devices such as a scan register, a BIST and a test pad that is contacted by a tester. The test transistor portion of the drive transistor is turned off 105, and if all test transistor portions of a driver transistor have not been tested 106, then the next test transistor portion is turned on 107, test current is coupled from the next test transistor portion to the current comparator circuit 103, and the process is continued.

If all test transistor portions of the driver transistor have been tested, then the switch coupling the test pad to the test bus is opened 109. If all driver transistors connected to an I/O pad have not been tested 110, the next output driver transistor is selected 111, a test transistor portion of the next output driver transistor is turned on 101 and the process continues. If all driver transistors connected to the I/O pad have been tested 112 and the last I/O pad has been tested for driver current 113, then end the test process. If the last I/O pad has not been tested for driver current 114, the next I/O pad is selected 115, a test transistor portion of the next I/O pad is turned on 101, and the process is continued.

In FIG. 6B is a method of the present invention for testing the current from a particular test transistor portion (M1 b, M2 b or M3 b) of an I/O driver transistor (M1, M2, or M3) first at all chip I/O pads containing the particular I/O driver transistor before continuing to the test transistor portion of the next I/O driver transistor. A bias voltage is connected to each of the I/O driver transistors on a semiconductor chip 120. A +V bias voltage is connected to the P-channel transistor M1 comprising transistor portions M1 a and M1 b, a −V bias voltage is connected to the N-channel transistor M2 comprising transistor portions M2 a and M2 b and a +Vm bias voltage is connected to the P-channel transistor M3 comprising transistor portions M3 a and M3 b. A test transistor portion (M1 b, M2 b or M3 b) is turned on 121 by control from the test logic 27, and the switch coupling the chip I/O pad to the test bus is closed 122. The current from the selected test transistor portion is coupled to a current comparator circuit and the current is compared to a reference current by the comparator circuit 123. The output of the comparator circuit is connected to an output mechanism 124 such as a test scan register, a BIST or a test pad to be contacted by tester. The selected test transistor portion of an I/O driver transistor is turned off 125 and if all test transistor portions of the selected driver transistor have been tested 128, then the switch connecting the Chip I/O pad to the test bus is opened 129. If all test transistor portions of the selected driver transistor have not been tested, then the next test transistor portion is turned on 127, current is coupled from the next test transistor portion to the current comparator circuit 123, and the process continues.

Continuing to refer to FIG. 6B, if the test transistor portions of last chip I/O pad has not been tested 130, then go to the next chip I/O pad 131 and turn on the test transistor portion connected to the chip I/O pad 121. Close the switch connecting the next chip I/O pad to the test bus 122, and continue the process. Continuing to refer to FIG. 6B, if the test transistor portion connected to the last chip I/O pad has been tested 132 and if all test transistor portions have been tested 133, then end. If all test transistor portions have not been tested 134 then turn on the next test transistor portion 135 connected to a chip I/O pad, and close the switch connecting the chip I/O pad to the test bus 122. Couple the on current from the next test transistor portion to the current comparator circuit, and compare the current of the next test transistor portion to a reference current 123, continue the process until all test transistor portions, comprising M1 b, M2 b and M3 b, connected to all Chip I/O pads have been tested for drive current capability.

Referring to the circuit diagram of FIG. 4E, a method of the present invention to measure the capability of an I/O input circuit 18 and 18 a to detect an input voltage is shown in FIG. 7. The bias (+V_(Logic) and −V_(Logic)) for the I/O input circuits is set to a specified test value 140. In the preferred embodiment of the present invention the logic bias −V_(Logic) and the bias −V for the N-channel driver transistor are logic ground; however, the method of FIG. 7 will function equally well if −V_(Logic) and −V have a voltage value that is negative with respect to circuit ground. A driver circuit bias is set to a voltage that equals the LPUL voltage of the input circuit 141, and a switch 21 is selected by the test logic, which connects a stand-alone I/O input circuit 18 a to the driver circuit 142. A driver transistor is selected to couple the LPUL voltage 143 to the input circuit 18 connected to the I/O pad 23 and the stand-alone input circuit 18 a. The logic circuits 19 connected to the outputs of the input circuits 18 and 18 a are polled to determine that the input circuits detected the LPUL voltage 144. The driver circuit bias is set to a LNDL voltage of the input circuits 145. The driver circuit is selected to couple the LNDL voltage to the input circuits 146, and the logic circuits 19 connected to the output of the input circuits are polled to determine that the input circuits detected the LNDL voltage 147.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention. 

1. A system for testing chip I/O (input/output) circuits, comprising: a) an integrated circuit chip; b) a first driver transistor and a second driver transistor to form an output driver circuit connected to an I/O pad of said chip; c) a test transistor formed from said first driver transistor; d) a test logic connected to a selector circuit to control selection of said test transistor to produce a test current; e) a switch circuit controlled by said test logic to couple said test current from said I/O pad to a current comparator circuit on said chip; and f) an output of said current comparator circuit to produce a test result by comparing the test current to a reference current.
 2. The system of claim 1 wherein said switch circuit couples test current from said I/O pad to a test bus to which is coupled a plurality of said I/O pads, whereby said test bus connects test current from said plurality of said I/O pads to the current comparator circuit.
 3. The system of claim 2, wherein said switch circuit further comprises: a) a current mirror circuit input transistor formed with the second driver transistor; b) a switch device controlled by the test logic connecting said input transistor to a mirror transistor of said current mirror circuit; and c) an output of said mirror transistor connected to the test bus to couple said test current from said first driver transistor to the current comparator circuit.
 4. The system of claim 3, wherein said mirror transistor is required for each said current mirror circuit that is connected to each said I/O pad of the chip.
 5. The system of claim 3, wherein said test bus is connected to a gate of the mirror transistor and the output of the mirror transistor is connected to the comparator circuit allowing a common said mirror transistor to be used with a plurality of said input transistors connected to a plurality of said I/O pads of the chip.
 6. The system of claim 3, wherein said current mirror circuit input transistor is formed with the first driver transistor when said second driver transistor is being controlled to produce said test current.
 7. The system of claim 1, wherein said test transistor is formed by segmenting a control gate area of said first driver transistor into a plurality of control gate portions from which test transistors of said first driver transistors are formed during test operations of said first transistor;
 8. The system of claim 7, wherein all said control gate portions of said first driver transistor is controlled by a driver logic to produce current from said output driver circuit during normal circuit operation.
 9. The system of claim 1, further comprising: a) a receiver circuit connected to said I/O pad; b) a bias voltage of said output driver circuit set to produce a marginal input signal level at input to said receiver circuit; and c) a logic circuit connected to the output of said receiver circuit determines capability of said receiver circuit to detect said marginal input signal level.
 10. The system of claim 9, wherein said output driver circuit produces said marginal input signal level and couples said marginal input signal level through a switch to test a remote receiver circuit not directly connected to said I/O pad and to said output driver circuit.
 11. The system of claim 1, wherein said test current is leakage current of said first and second driver transistor.
 12. The system of claim 1, wherein said test current is “on” current of said test transistor.
 13. The system of claim 1, wherein said switch circuit couples current from said output driver circuits to the current comparator connected to said test bus, in which said test bus is further coupled to a plurality of said switch circuits whereby a plurality of said output driver circuits are coupled to the current comparator to allow current of each output driver circuit of said plurality of output drivers circuits to be measured by said current comparator.
 14. The system of claim 13, wherein said current of each said output driver circuit is measured by individually closing each said switch circuit of said plurality of switch circuits thereby coupling current from each output driver circuit of said plurality of output driver circuits to the current comparator.
 15. The system of claim 13, wherein said plurality of switches are closed simultaneously by said test logic to measure a collective sum of leakage current from said plurality of output driver circuits coupled to said scan bus.
 16. The system of claim 1, wherein said test current is coupled to a test pad connected to said I/O pad to allow measurement of said test current by test equipment.
 17. The system of claim 1, wherein said test result of the current comparator is read out by various devices, comprising: a) a scan register; b) a BIST (built-in self test) circuit; and c) test equipment.
 18. A method to test an I/O input circuit, comprising: a) setting a bias voltage to a specified test value for an I/O input circuit connected to an I/O pad; b) setting said bias voltage for a driver transistor of an output driver circuit connected to said I/O pad to a least positive up level (LPUL) voltage of an input signal for said I/O input circuit; c) selecting said driver transistor to couple said LPUL voltage to said I/O input circuit; d) polling said logic connected to an output of the I/O input circuit to determine said LPUL voltage detected by the I/O input circuit; e) setting said bias voltage for said driver transistor of said output driver circuit connected to said I/O pad to a least negative down level (LNDL) voltage of said input signal for said I/O input circuit; f) selecting said driver transistor to couple said LNDL voltage to said I/O input circuit; and g) polling said logic connected to said output of the I/O input circuit to determine said LNDL voltage detected by the I/O input circuit.
 19. The method of claim 18, further comprising a closing of a switch connected to said I/O pad to connect stand-alone I/O input circuits to the LPUL and LNDL voltages.
 20. The method of claim 19, wherein said logic connected to the output of the stand-alone I/O input circuits is polled to determine said LPUL and LNDL voltages are detected. 